The most fundamental property of a star is its mass.  We have seen how mass dictates the evolution of a star, controlling its lifetime, end state, and which elements it makes by fusion.  So how do we measure mass?  The most fundamental way is to use binary stars: two stars orbiting each other under their mutual gravitational attraction.  By observing properties of the binary such as the orbital period and the separation of the stars, we can learn about the masses of the stars.  About half of the points of light that we see when we look up into the night sky are not single stars like our Sun but rather the combined light from two or more stars orbiting each other.  For example, the three closest stars to our Sun (remember the parallax and H-R Diagram labs?), alpha Centauri, alpha Centauri-B, and Proxima Centauri are an example of a triple system.

In this lab you will find out how astronomers learn about stars by studying binary stars. You will also learn how astronomers find extrasolar planets using techniques that are similar to those they use to observe binary stars. You will use some of these techniques to learn about various properties of stars and extrasolar planets.


  1. Read the background material by clicking on the topics listed on the menu on the left side of this page. For a given topic there may be multiple pages each of which can be accessed using the navigation buttons arranged across the top of each page.

  2. Click on the menu item "Problems". A window should open at the bottom of your screen with the questions.

  3. Use the interactive exercise on eclipsing binaries to learn how photometric light curves can tell us about the properties of stars in binary systems.

  4. Use the interactive exercise on spectroscopic binaries to learn how radial velocity light curves can teach us about the properties of stars in binary systems.

  5. Become familiar with the extrasolar planet catalog and learn how to use it to get information on extrasolar planets.

  6. Answer the questions by filling in the form fields on the computer. Hit the button labeled "Submit" when you are done. A confirmation page will appear with all your answers imbedded. Be careful not to close the problems window until you are finished and want to submit or your answers may be lost and you will have to reenter your answers.

  7. Check over the confirmation page to make sure that you are happy with your answers. If you need to change your answers go back to step 6 and re-submit. We will only grade the last form you submit.

    Good Luck!